Rule303 has told you well. With the understanding that the .45-70 comparison load is the common 300 gn HP or Softpoint at about 1870 from Win/Rem/Fed then yes, modern sabot 20ga loads fly and hit like the .45-70.
Here's the "But" though. The 300gn rifle loads are 40 to 20 grains heavier so they have about a 10% better BC and sectional density and will fly a bit flatter and penetrate better on shots that angle through the body. Two five round boxes of sabot slugs cost about what a 20rd box of .45-70 does or what 4 boxes of 20 ga foster style slugs do, so "more bangs for the buck" with these and LESS with the 'rifled 20ga'. The smoothbore style 20ga slug barrel doubles with buckshot for defense better and comes in shorter lengths than the rifled tubes. Pump shotguns are long overall and don't handle so well with 24 to 28 inch 'birdshot' or rifled barrels but there's no problem finding 18-22 inch smooth/unrifled barrels. The 20in and less smooth barrel handles so much better moving around a room or in the brushy short visibility lands that are often found in "shotgun" only areas.
While you might not be worried about it, remember that the sabot slug is a low volume market for the ammo company, and it's getting lower. The "Great Lakes" states are a MAJOR part of the land area that is shotgun only (in some of these states thay had been exclusively shotgun only). The more these states allow a restricted use of rifles (like IN, and now this year OH and MI also) or pistol or "blackpowder rifles" in the regular gun deer season the fewer guys buying sabot shotgun shells. Look for the price of sabots to become even more ridiculous and than begin disappearing as they are loss leaders for the companies.
As to reloading, there are some components available. Most of these are of the foster/improved foster design (like the newer Lyman and Lee drive key and "pellet" style slugs--really just modified fosters). A "buckhammer" style is available. My opinion of the 'attached wad' style slug is it falls in the foster camp performance wise. Sabots to hold a jacketed pistol/rifle style bullet are about non existent in 20 ga. Not so in 12, but still very limited and lab tested load data is MINIMAL.